Hi all..my '86 with a 2.8 TBI will idle fine on start-up but after 20-30 secs it starts to fluctuate, goes from about 800 rpm up to 1700 then drops back, does this continuosly. I've replaced the fuel pump, filter and pressure regulator and this problem started 2 days after that, ran fine at first. I'm pretty sure it's a bad sensor but haven't a clue which one...any tips would be greatly appreciated.

 

check the crankshaft positioning sensor it could be that the motor is runnin all over the place cause it doesn't know were its at try that.

 

I'd check to see if the 2.8L actually had a CKP sensor first...

How much time is there at either RPM or is it continuously fluctuating?

 

Hi neighbor, relativly speaking.
The first thing I'd be looking for is a vacuum leak.

 

Throttle position sensor? Idle air control valve? Vacuum leak? Is the 86 carbed or fuel injected anyway? Nevermind, I'm retarded.. he said TBI in the post.

 

Yeah, '86 was the first year of the TBI 2.8L. I should know, I had a stupid feedback carb'd 2.8L '85 S10. What a piece that setup was!

I concure on the vacuum leak. You can check for a vacuum leak with an unlit propane torch and the engine running. You'll know you've found it when the rpm's pickup and stay up. Area's of interest should be any vacuum lines as well as the TBI base gasket.

 

I've never had the pleasure of working on the 2.8L. but from what I've heard they aren't one of GM's better designs. I know they were in the Isuzu Troopers at one point, I almost bought one a couple years ago.. From what I've heard, I'm glad I didn't

 

They are good engines if you aren't in a hurry to get anywhere. The 2.8L TBI motor in my grandpa's old '89 S10 is still going strong for the new owner at over 275k miles (mileage when we sold it) without anything going wrong with the internals of the engine. But they do seem to be plagued with little annoyances that are quite difficult to solve.

 

Little annoyances. Thats how my GM mechanic friend put it. LOL. I never understood why they didn't put the 4.3s in the Troopers. Those things have awesome 4 wheel capabilities, but the underpowered engines are unbearable. I drove a 88 I think, 4 cylinder 5 speed. Good lord....

 

Thanks all for the tips...the '86 doesn't have a CPS so that rules that out. But I did find what it was eventually by reading the factory manual..lol..who needs a stinkin' manual. I pulled the fuses on the ECM to clear up a code I deliberately set just to make sure it was working, I disconnected the MAP sensor. Turns out that after you do this you need to run the truck up to operating temp. at an easy throttle for 10-15 mins. until the ECM goes into open loop and this resets all the sensor inputs. It seems that the ECM cannot control the IAC valve after the power is off and needs to warm up first. Now if I can just figure out why my injectors are dripping. If anyone has any ideas, read my other post and your 2 cents would be greatly appreciated.